[flang] Inline array size call when dim is compile time constant

Instead of calling _FortranASizeDim, we can instead load extent
directly from descriptor. Add this support for cases where dim
is a known constant at compile time.

Reviewed By: clementval

Differential Revision: https://reviews.llvm.org/D150385

1 files changed, 7 insertions, 0 deletions

diff --git a/flang/lib/Optimizer/Builder/IntrinsicCall.cpp b/flang/lib/Optimizer/Builder/IntrinsicCall.cpp
index dee08c7d85f8..2ee6f404ceef 100644
--- a/flang/lib/Optimizer/Builder/IntrinsicCall.cpp
+++ b/flang/lib/Optimizer/Builder/IntrinsicCall.cpp
@@ -4907,6 +4907,13 @@ IntrinsicLibrary::genSize(mlir::Type resultType,
 
   // Get the DIM argument.
   mlir::Value dim = fir::getBase(args[1]);
+  if (std::optional<std::int64_t> cstDim = fir::getIntIfConstant(dim)) {
+    // If it is a compile time constant, skip the runtime call.
+    return builder.createConvert(loc, resultType,
+                                 fir::factory::readExtent(builder, loc,
+                                                          fir::BoxValue{array},
+                                                          cstDim.value() - 1));
+  }
   if (!fir::isa_ref_type(dim.getType()))
     return builder.createConvert(
         loc, resultType, fir::runtime::genSizeDim(builder, loc, array, dim));